This invention relates in general to an active network for use as a build out capacitor circuit, and in particular, to a circuit for producing a variable virtual capacitance from an active network using fixed and variable resistances. In VF telephone transmission a build-out capacitor is frequently used to build-out an end section of a loaded transmission line. Such loading schemes, which are well known in the prior art, are used to extend the useable distance between a central office and a subscriber.
In telephone systems, the ideal near end section length and far end section length, i.e., the length of transmission line between central office and a first loading coil and the length of transmission line between a last loading coil and a subscriber, respectively, is equal to one half of the nominal load coil spacing. Such configuration would require the total length of transmission line between the central office and the subscriber to be a multiple of coil spacing. Since a telephone company has no control over the distance between the central office and the subscriber, the near end section can have any length between zero and full section spacing.
A shunt capacitor, referred to as a Build Out Capacitor (BOC) terminates the cable pair at the central office side and "builds out" the near end section to a full length section. The telephone line must be balanced so that the impedance between the tip terminal and ground is the same as the impedance between the ring terminal and ground. Any imbalance would cause a substantial noise pickup from adjacent power lines or other sources of undesired signals. The construction and operation of such a structure is well known in the prior art.
Standard practice is to use build out capacitors in which a large number of capacitors in binary progression are switched in and out in various combinations to achieve the desired value.
For N capacitor switch combinations, 2.sup.N build out capacitor combination values could be implemented. Such a method of implementing build out capacitance is expensive and space consuming. It is also not practical for logic control or other forms of electronic control
The present invention overcomes the problem in the prior art of using a large number of individual capacitors. A novel compact active network produces a variable capacitance to replace the prior art circuits.